The influence of maternal body mass index on myometrial oxytocin receptor expression in pregnancy

The Interplay of Uterine Health and Obesity: A Comprehensive Review

Uterine physiology encompasses the intricate processes governing the structure, function, and regulation of the uterus, a pivotal organ within the female reproductive system. The escalating prevalence of obesity has emerged as a significant global health issue, profoundly impacting various facets of well-being, including female reproductive health. These effects extend to uterine structure and function, influencing reproductive health outcomes in women. They encompass alterations in uterine morphology, disruptions in hormonal signaling, and inflammatory processes. Insulin and leptin, pivotal hormones regulating metabolism, energy balance, and reproductive function, play crucial roles in this context. Insulin chiefly governs glucose metabolism and storage, while leptin regulates appetite and energy expenditure. However, in obesity, resistance to both insulin and leptin can develop, impacting uterine function. Inflammation and oxidative stress further exacerbate the development of uterine dysfunction in obesity. Chronic low-grade inflammation and heightened oxidative stress, characteristic of obesity, contribute to metabolic disruptions and tissue damage, including within the uterus. Obesity significantly disrupts menstrual cycles, fertility, and pregnancy outcomes in women. The accumulation of excess adipose tissue disrupts hormonal equilibrium, disturbs ovarian function, and fosters metabolic irregularities, all of which detrimentally impact reproductive health.

Maternal Body Mass Index, Myometrium Contractility and Uterotonic Receptor Expression in Pregnancy

Pregnant individuals with obesity (body mass index, BMI ≥ 30 kg/m2) are more likely to experience prolonged labor and have double the risk of cesarean compared with individuals with normal weight (BMI < 25 kg/m2). The aim of this study was to evaluate whether obesity in pregnancy is associated with reduced spontaneous and oxytocin-stimulated myometrial contractile activity using ex vivo preparations. We also assessed the relationship between maternal BMI and the expression of oxytocin (OXTR) and prostaglandin (FP) receptors in the myometrial tissue. We enrolled 73 individuals with a singleton gestation undergoing scheduled cesarean delivery at term in a prospective cohort study. This included 49 individuals with a pre-pregnancy BMI ≥ 30 kg/m2 and 24 with BMI < 25.0 kg/m2. After delivery, a small strip of myometrium was excised from the upper edge of the hysterotomy. Baseline spontaneous and oxytocin stimulated myometrial contractile activity was measured using ex vivo preparations. Additionally, expression of oxytocin and prostaglandin receptors from myometrial samples were compared using qRT-PCR and western blot techniques. Spontaneous and oxytocin-stimulated contraction frequency, duration, and force were not significantly different in myometrial samples from the obese and normal-weight individuals. Myometrial OXTR gene and protein expression was also similar in the two groups. While FP gene expression was lower in the myometrial samples from the obese group, protein expression did not differ. These data help to address an important knowledge gap related to the biological mechanisms underlying the association between maternal obesity and dysfunctional labor.

The minimum effective dose (ED90) of prophylactic oxytocin infusion during cesarean delivery in patients with and without obesity: an up-down sequential allocation dose-response study

BACKGROUND

Obesity is associated with greater oxytocin requirement during labor induction or augmentation. There are scant data exploring the intra-operative requirement during cesarean delivery in patients with obesity, and none comparing it with those without obesity. We evaluated the minimum effective dose (ED90) of an oxytocin infusion to achieve adequate uterine tone during cesarean delivery in patients with and without obesity.

METHODS

Patients (body mass index ≥30 kg/m2 represented patients with obesity) undergoing cesarean delivery using subarachnoid block were included. This prospective dual-arm dose-finding study used a 9:1 biased sequential allocation design. Oxytocin infusion was initiated at 13 IU/h at cord clamping in the first patient of each group. Uterine tone was graded as satisfactory or unsatisfactory by the obstetrician four minutes after initiation of the infusion. The dose of oxytocin infusion for subsequent patients was determined according to the response of the previous patient in the group. Oxytocin-associated side effects were evaluated. Dose-response data for the groups was evaluated using log-logistic function and ED90 estimates derived from fitted equations using the delta method.

RESULTS

The ED90 of oxytocin was significantly higher for patients with obesity (n = 40) compared with those without obesity (n = 40) [25.7 IU/h, 95% CI 18.6 to 32.9) vs. 16.6 IU/h, 95% CI 14.9 to 18.3)]; relative ratio 1.55 [95% CI 1.09 to 2.01] (P = 0.019).

CONCLUSIONS

Patients with obesity require a higher intra-operative oxytocin infusion dose rate to achieve a satisfactorily contracted uterus after fetal delivery when compared with patients without obesity.

Ubhayasekera K, Kjölhede P, Bergquist J, Blomberg M. The association between maternal body mass index and serial plasma oxytocin levels during labor

OBJECTIVE

To evaluate the association between maternal body mass index (BMI) and plasma oxytocin (OT) levels at different OT infusion rates in labor.

METHODS

A prospective observational study analyzing serial plasma samples in laboring women with OT infusion. The women were categorized into three groups, women with non-obesity (BMI 18.5-29.9, n = 12), obesity (BMI 30.0-34.9, n = 13), and morbid obesity (BMI ≥ 35.0, n = 15). Plasma OT was analyzed using tandem mass spectrometry.

RESULTS

Except for a low positive correlation between OT levels and BMI and significantly increased plasma OT levels in women with morbid obesity at the OT infusion rate of 3.3 mU/min, no significant differences in OT levels between the BMI groups were found. Further, the inter-individual differences in OT levels were large and no dose-dependent increase of OT levels was seen.

CONCLUSIONS

Other factors than plasma OT levels may be more likely to determine the clinical response of OT infusion in women with obesity. Perhaps the observed clinical need and individual response would be a better predictor of plasma OT levels than a pre-determined OT infusion rate. The OT dosage guidelines for labor augmentation should be individualized according to clinical response rather than generalized.

TRIAL REGISTRATION

Clinical trial registration: ClinicalTrials.gov ID NCT04093479.

The Pathophysiology of Labor Dystocia: Theme with Variations

Abnormally prolonged labor, or labor dystocia, is a common complication of parturition. It is the indication for about half of unplanned cesarean deliveries in low-risk nulliparous women. Reducing the rate of unplanned cesarean birth in the USA has been a public health priority over the last two decades with limited success. Labor dystocia is a complex disorder due to multiple causes with a common clinical outcome of slow cervical dilation and fetal descent. A better understanding of the pathophysiologic mechanisms of labor dystocia could lead to new clinical opportunities to increase the rate of normal vaginal delivery, reduce cesarean birth rates, and improve maternal and neonatal health. We conducted a literature review of the causes and pathophysiologic mechanisms of labor dystocia. We summarize known mechanisms supported by clinical and experimental data and newer hypotheses with less supporting evidence. We review recent data on uterine preparation for labor, uterine contractility, cervical preparation for labor, maternal obesity, cephalopelvic disproportion, fetal malposition, intrauterine infection, and maternal stress. We also describe current clinical approaches to preventing and managing labor dystocia. The variation in pathophysiologic causes of labor dystocia probably limits the utility of current general treatment options. However, treatments targeting specific underlying etiologies could be more effective. We found that the pathophysiologic basis of labor dystocia is under-researched, offering wide opportunities for translational investigation of individualized labor management, particularly regarding uterine metabolism and fetal position. More precise diagnostic tools and individualized therapies for labor dystocia might lead to better outcomes. We conclude that additional knowledge of parturition physiology coupled with rigorous clinical evaluation of novel biologically directed treatments could improve obstetric quality of care.

Induction of labour in pregnant individuals with obesity

People with obesity may require induction of labour (IoL) due to a higher incidence of pre-existing comorbidities and pregnancy complications, as well as to prevent post-term pregnancies and late-term stillbirths. IoL at 39-40 weeks is associated with fewer caesarean births and lower morbidity for the pregnant person and neonate when compared with expectant management. Ensuring the success and safety of IoL in people with obesity requires adherence to evidence-based protocols for the management of labour induction and augmentation. Cervical ripening as well as the latent and active phases of labour in people with obesity may be considerably prolonged, requiring higher cumulative doses of oxytocin. This should be guided by intrauterine pressure catheters and early provision of neuraxial analgesia, where possible. There is insufficient evidence to recommend one method of IoL over another. The need for higher doses of prostaglandins and concurrent agents for cervical ripening should be studied in prospective studies.

The Myometrium in Pregnant Women with Obesity

Obesity is a worldwide public health problem, affecting at least one-third of pregnant women. One of the main problems of obesity during pregnancy is the resulting high rate of cesarean section. The leading cause of this higher frequency of cesarean sections in obese women, compared with that in nonobese women, is an altered myometrial function that leads to lower frequency and potency of contractions. In this article, the disruptions of myometrial myocytes were reviewed in obese women during pregnancy that may explain the dysfunctional labor. The myometrium of obese women exhibited lower expression of connexin43, a lower function of the oxytocin receptor, and higher activity of the potassium channels. Adipokines, such as leptin, visfatin, and apelin, whose concentrations are higher in obese women, decreased myometrial contractility, perhaps by inhibiting the myometrial RhoA/ROCK pathway. The characteristically higher cholesterol levels of obese women alter myometrial myocyte cell membranes, especially the caveolae, inhibiting oxytocin receptor function, and increasing the K+ channel activity. All these changes in the myometrial cells or their environment decrease myometrial contractility, at least partially explaining the higher rate of cesarean of sections in obese women.

Use of the Robson classification to understand the increased risk of cesarean section in case of maternal obesity

BACKGROUND

The aim of this study was to identify characteristics of pregnant women with obesity that contribute to increased cesarean rate.

METHODS

Retrospective cohort in a single academic institution between 2012 and 2019. Women who delivered during this period were classified according to the Robson classification. Women with normal body mass index (N = 11,797) and with obesity (N = 2991) were compared. The contribution of each Robson group to the overall caesarean rate were compared.

RESULTS

The overall cesarean rate was higher for women with (28.1%) than without (14.2%, p < 0.001) obesity. This result came mainly from Robson group 5a (history of one cesarean). After adjustment for medical factors within this group, the association between maternal obesity and cesarean during labor was significant.

CONCLUSIONS

The higher cesarean rate in women with obesity is explained by Robson group 5a in which obesity is an independent risk factor of in labor cesarean delivery.

The Role of Oxytocin in Primary Cesarean Birth Among Low-Risk Women

INTRODUCTION

To examine whether there is a threshold of oxytocin exposure at which the risk for primary cesarean increases among women who are nulliparous with a term, singleton, vertex fetus (NTSV) and how oxytocin interacts with other risk factors to contribute to this outcome.

METHODS

This was a secondary analysis of the Consortium on Safe Labor data set that used a retrospective cohort study design. Women who met the criteria for NTSV who were not admitted for a prelabor cesarean and for whom oxytocin data were available, were included in the sample. Robust logistic regression was used to examine the association of oxytocin exposure with primary cesarean birth, while controlling for demographic and clinical risk factors and clustering by provider.

RESULTS

The sample comprised 17,331 women who were exposed to oxytocin during labor. The women were predominantly white non-Hispanic (59.2%) with an average (SD) gestational age of 39.4 (1.1) weeks and an 18.5% primary cesarean rate. Exposure to greater than 11,400-milliunits (mU) of oxytocin resulted in 1.6 times increased odds of primary cesarean birth compared with less than 11,400 mU (95% CI 1.01-2.6).

DISCUSSION

Exposure to greater than 11,400 mU of oxytocin in labor was associated with an increased odds of primary cesarean birth in NTSV women.

Risk factors for failed induction of labor among pregnant women with Class III obesity

INTRODUCTION

Our aim was to identify risk factors for failed induction in morbidly obese patients undergoing the induction of labor at term.

MATERIAL AND METHODS

This was a retrospective multicenter study on a cohort of 235 patients with a body mass index greater than 40 kg/m² and giving birth to a singleton in cephalic presentation, who had an induction of labor from 38 weeks of amenorrhea. Scheduled cesareans and spontaneous vaginal deliveries were excluded. Maternal, peri-partum and neonatal characteristics were analyzed according to the delivery route.

RESULTS

In all, 235 patients were included. Of these, 62.5% patients delivered vaginally and 37.5% by cesarean section. The frequency of nulliparity was greater in patients who had a cesarean section (56 [interquartile range, IQR, 38.1] vs 56 [IQR 63.6], P < .001). In multivariate analysis, nulliparity (odds ratio [OR] 2.81, 95% confidence interval [CI] 1.58-4.97], P < .001), low Bishop's score (OR .794, 95% CI .70-.90, P < .001) and weight gain (OR 1.04, 95% CI 1.01-1.08, P = .033) were independent risk factors for failed induction. Umbilical cord pH at birth lower than 7 (0 vs 7 [IQR 8.0], P < .001) and lower than 7.20 (36 [IQR 24.5] vs 35 [IQR 39.8], P = .014) as well as the Apgar at 1 minute (14 [IQR 9.5] vs 17 [IQR 19.3], P = .032) was significantly higher in infants born by cesarean section.

CONCLUSIONS

In this cohort, 63% of women with Class III obesity had successful inductions of labor; risk factors for failed induction include nulliparity and unfavorable Bishop score.

Effect of Maternal Body Mass Index on Postpartum Hemorrhage

BACKGROUND

It is unclear whether obesity is a risk factor for postpartum hemorrhage. The authors hypothesized that obese women are at greater risk of hemorrhage than women with a normal body mass index.

METHODS

The authors conducted a cohort study of women who underwent delivery hospitalization in California between 2008 and 2012. Using multilevel regression, the authors examined the relationships between body mass index with hemorrhage (primary outcome), atonic hemorrhage, and severe hemorrhage (secondary outcomes). Stratified analyses were performed according to delivery mode.

RESULTS

The absolute event rate for hemorrhage was 60,604/2,176,673 (2.8%). In this cohort, 4% of women were underweight, 49.1% of women were normal body mass index, 25.9% of women were overweight, and 12.7%, 5.2%, and 3.1% of women were in obesity class I, II, and III, respectively. Compared to normal body mass index women, the odds of hemorrhage and atonic hemorrhage were modestly increased for overweight women (hemorrhage: adjusted odds ratio [aOR], 1.06; 99% CI, 1.04 to 1.08; atonic hemorrhage: aOR, 1.07; 99% CI, 1.05 to 1.09) and obesity class I (hemorrhage: aOR, 1.08; 99% CI, 1.05 to 1.11; atonic hemorrhage; aOR, 1.11; 99% CI, 1.08 to 1.15). After vaginal delivery, overweight and obese women had up to 19% increased odds of hemorrhage or atonic hemorrhage; whereas, after cesarean delivery, women in any obesity class had up to 14% decreased odds of severe hemorrhage.

CONCLUSIONS

The authors' findings suggest that, at most, maternal obesity has a modest effect on hemorrhage risk. The direction of the association between hemorrhage and body mass index may differ by delivery mode.

A conceptual framework for the impact of obesity on risk of cesarean delivery

Cesarean deliveries accounted for 32.2% of nearly 4 million births in the United States in 2014. Obesity affects a third of reproductive-age women and is associated with worse cesarean delivery outcomes. Studies have shown that increasing maternal body mass index correlates linearly with cesarean delivery rates, but little is known about the potential mediating and moderating mechanisms. Thus, a conceptual framework for understanding how obesity correlates with risk of cesarean delivery is crucial to determining safe ways to reduce the cesarean delivery rate among obese gravidas. Based on an extensive review and synthesis of the literature, we present a conceptual framework that posits how obesity may operate through several pathways to lead to a cesarean delivery. Our framework explores the complexity of obesity as an exposure that operates through potential mediating pathways, a moderator of cesarean delivery risk, and a covariate with other cesarean delivery risk factors. Among nulliparas, obesity appears to operate through 3 main proximal mediating mechanisms to increase risk of cesarean delivery including: (1) preexisting comorbidities and obstetric complications; (2) a slower progression of first-stage labor, potentially increasing the risk of cesarean delivery secondary to failure to progress; and (3) a prolongation of pregnancy, which is associated with risk of maternal postdates. For multiparas, a fourth proximal mediator of prior uterine scar may also increase cesarean delivery risk. Distal mediating mechanisms, which operate through one of the proximal mechanisms, may include an induction of labor or planned prelabor cesarean delivery. Obesity may also moderate the likelihood of cesarean delivery by interacting with clinician-level or hospital-level factors. Future research should assess the validity of this framework and seek to understand the relative contributions of each potential pathway between obesity and cesarean delivery. This will allow for evidence-based recommendations to reduce preventable cesareans among obese women by targeting modifiable mediators and moderators of the relationship between obesity and increased risk of cesarean delivery.

Child Maltreatment Is Associated with a Reduction of the Oxytocin Receptor in Peripheral Blood Mononuclear Cells

Background: Child maltreatment (CM) and attachment experiences are closely linked to alterations in the human oxytocin (OXT) system. However, human data about oxytocin receptor (OXTR) protein levels are lacking. Therefore, we investigated oxytocin receptor (OXTR) protein levels in circulating immune cells and related them to circulating levels of OXT in peripheral blood. We hypothesized reduced OXTR protein levels, associated with both, experiences of CM and an insecure attachment representation.

Methods: OXTR protein expressions were analyzed by western blot analyses in peripheral blood mononuclear cells (PBMC) and plasma OXT levels were determined by radioimmunoassay (RIA) in 49 mothers. We used the Childhood Trauma Questionnaire (CTQ) to assess adverse childhood experiences. Attachment representations (secure vs. insecure) were classified using the Adult Attachment Projective Picture System (AAP) and levels of anxiety and depression were assessed with the German version of the Hospital Depression and Anxiety scale (HADS-D).

Results: CM-affected women showed significantly lower OXTR protein expression with significantly negative correlations between the OXTR protein expression and the CTQ sum score, whereas plasma OXT levels showed no significant differences in association with CM. Lower OXTR protein expression in PBMC were particularly pronounced in the group of insecurely attached mothers compared to the securely attached group. Anxiety levels were significantly higher in CM-affected women.

Conclusion: This study demonstrated a significant association between CM and an alteration of OXTR protein expression in human blood cells as a sign for chronic, long-lasting alterations in this attachment-related neurobiological system.

Unchanged mitochondrial phenotype, but accumulation of lipids in the myometrium in obese pregnant women

KEY POINTS

Obesity during pregnancy and childbirth is associated with labour dystocia leading to instrumental or operative delivery, but the underlying pathophysiological mechanisms remain unclear and insufficient uterine contractility has been suggested. This study examined whether reduced myometrial mitochondrial capacity or quantity could contribute as a pathophysiological mechanism to labour dystocia. Data did not support reduced myometrial mitochondrial capacity or quantity in the myometrium at term in obese women, but a reduced myocyte density with increased triglyceride content was demonstrated, which could lead to poorer uterine contractility. These results add to the understanding of systemic effects of obesity, placing also the myometrium at term as an affected non-adipose tissue.

ABSTRACT

Obesity is known to increase the risk of labour dystocia and insufficient energy supply, due to reduced mitochondrial capacity or quantity, could be a possible mechanism leading to reduced efficiency of uterine contractility during labour. In the present study of 36 women having an elective Caesarean section at term, obesity did not change mitochondrial phenotype in the myometrial myocyte obtained from uterine biopsies taken at delivery. Respiration rates in isolated mitochondria were unaffected by obesity. No indication of reduced content, investigated by quantification of the complexes of the respiratory chain, or altered regulation, examined by myometrial mRNA levels of genes related to mitochondrial biogenesis and inflammation, was detected. Yet we found increased myometrial triglyceride content in the obese group (2.39 ± 0.26 vs. 1.56 ± 0.20 mm, P = 0.024), while protein content and citrate synthase activity per gram wet weight myometrium were significantly lower in the obese (109.2 ± 7.2 vs. 139.4 ± 5.6 mg g^-1 , P = 0.002, and 24.8 ± 1.0 vs. 29.6 ± 1.4 U g^-1 wet wt, P = 0.008, respectively). These differences were substantiated by our histological findings where staining for nuclei, cytoplasm, glycogen and collagen supported the idea of a smaller muscle content in the myometrium in obese women. In conclusion no indication of myometrial mitochondrial dysfunction in the isolated state was found, but the observed increase of lipid content might play a role in the pathophysiological mechanisms behind labour dystocia in obese women.

The effect of obesity on early fetal growth and pregnancy duration: a cohort study

OBJECTIVE

The objective of this study is to investigate the effect of maternal obesity on fetal size in first- and second-trimester pregnancies and to determine duration of pregnancy as estimated by a variety of methods.

METHODS

Between 2006 and 2011, a cohort study included (n = 9055) singleton pregnancies that resulted in live birth at Holbaek Hospital in Denmark. This study recorded first- and second-trimester fetal measurements and maternal anthropometry. Characteristics considered included mother's age, parity, height, body mass index (BMI), smoking habits, and sex of child. The correlation between BMI and duration of pregnancy was analyzed by time-to-event analysis and accounted for medical intervention by censoring while correlation of BMI on fetal size was evaluated by multiple regression analysis.

RESULTS

Adjusting for maternal and fetal characteristics, BMI was associated with prolonged pregnancy duration (0.20-0.22 d per kg/m² (standard error (SE) 0.02)) when using ultrasound and 0.26 d per kg/m² (SE: 0.03) when using last menstrual period. With increasing BMI, fetal biometries in first and second trimester were significantly smaller than expected (0.08 mm per kg/m² when measured by crown rump length (SE 0.02)).

CONCLUSIONS

Maternal BMI is correlated to smaller fetal size in early pregnancy and prolongs duration of pregnancy.

Titration of Intravenous Oxytocin Infusion for Postdates Induction of Labor Across Body Mass Index Groups

OBJECTIVE

To evaluate whether oxytocin titration for postdates labor induction differs among women who are normal weight, overweight, and obese and whether length of labor and birth method differ by oxytocin titration and body mass index (BMI).

DESIGN

Retrospective cohort study.

SETTING

U.S. university-affiliated hospital.

PARTICIPANTS

Of 280 eligible women, 21 were normal weight, 134 were overweight, and 125 were obese at labor admission.

METHODS

Data on women who received oxytocin for postdates induction between January 1, 2013 and June 30, 2013 were extracted from medical records. Oxytocin administration and labor outcomes were compared across BMI groups, controlling for potential confounders. Data were analyzed using χ², analysis of variance, analysis of covariance, and multiple linear and logistic regression models.

RESULTS

Women who were obese received more oxytocin than women who were overweight in the unadjusted analysis of variance (7.50 units compared with 5.92 units, p = .031). Women who were overweight had more minutes between rate changes from initiation to maximum than women who were obese (98.19 minutes compared with 83.39 minutes, p = .038). Length of labor increased with BMI (p = .018), with a mean length of labor for the normal weight group of 13.96 hours (standard deviation = 8.10); for the overweight group, 16.00 hours (standard deviation = 7.54); and for the obese group, 18.30 hours (standard deviation = 8.65). Cesarean rate increased with BMI (p = .001), with 4.8% of normal weight, 33.6% of overweight, and 42.4% of obese women having cesarean births.

CONCLUSION

Women who were obese and experienced postdates labor induction received more oxytocin than women who were non-obese and had longer length of labor and greater cesarean rates.

Maternal body mass index and spontaneous contractility of human myometrium in pregnancy

OBJECTIVE

There is controversy as to whether maternal body mass index (BMI) influences the contractility of human myometrium in pregnancy. The aim of this study was to examine spontaneous contractile activity of human pregnant myometrium in vitro, with respect to maternal BMI.

STUDY DESIGN

Myometrial tissue specimens were obtained at cesarean delivery from 74 women with BMI values ranging from 19 to 50.1 kg m^-2. By recording in vitro from eight strips per donor (590 strips in total), several parameters of spontaneous contractile activity were monitored. The relationship between BMI and contractility was evaluated using linear regression analysis.

RESULTS

There was a significant correlation between maximum amplitude (P=0.007) and mean contractile force (P=0.001) with increasing BMI. However, the time to onset of contractions (P=0.009), and time taken to reach maximal amplitude (P=0.020) also increased with increasing BMI. No significant correlation was observed with BMI for other parameters studied. The mean maximum amplitude value for spontaneous contractions was 37±1 mN, the mean contractile force for spontaneous contractions was 4.1±0.1 mN, the average time to the first spontaneous contraction was 11.3±0.6 min and the average frequency of contractions was 6.5±0.2 per hour.

CONCLUSIONS

These results suggest that the time to onset of contractions is increased with increasing maternal BMI, but that the force developed is greater. In all other respects, human uterine contractility is unaffected by increasing BMI. These findings underline the complexity of regulation of uterine contractility in labor with elevated maternal BMI.

Intrapartum Management of the Obese Gravida

Obese women are at increased risk for multiple labor abnormalities, including postdates pregnancy, failed induction of labor, prolonged labor, cesarean delivery, and postpartum hemorrhage (PPH). Prolonged labor among obese women is confined to the first stage of labor. In the setting of reassuring fetal and maternal status, increased time to progress in the first stage of labor should be allowed. Uterine atony occurs more frequently in obese women and vigilance in the prevention of PPH is critical. There is a lack of high-quality data to guide the management of induction, labor, and PPH prevention among obese women.

Parturition dysfunction in obesity: time to target the pathobiology

Over a third of women of childbearing age in the United States are obese, and during pregnancy they are at increased risk for delayed labor onset and slow labor progress that often results in unplanned cesarean delivery. The biology behind this dysfunctional parturition is not well understood. Studies of obesity-induced changes in parturition physiology may facilitate approaches to optimize labor in obese women. In this review, we summarize known and proposed biologic effects of obesity on labor preparation, contraction/synchronization, and endurance, drawing on both clinical observation and experimental data. We present evidence from human and animal studies of interactions between obesity and parturition signaling in all elements of the birth process, including: delayed cervical ripening, prostaglandin insensitivity, amniotic membrane strengthening, decreased myometrial oxytocin receptor expression, decreased myocyte action potential initiation and contractility, decreased myocyte gap junction formation, and impaired myocyte neutralization of reactive oxygen species. We found convincing clinical data on the effect of obesity on labor initiation and successful delivery, but few studies on the underlying pathobiology. We suggest research opportunities and therapeutic interventions based on plausible biologic mechanisms.

Induction of labor in the obese patient

Obese women are at an increased risk of antepartum pregnancy complications and are therefore more likely to require an induction of labor than normal weight women. They also have an increased rate of failing an induction of labor, a rate that rises significantly with increasing body mass index, and subsequent surgical and neonatal complications of an intrapartum cesarean delivery. This increase in induction failure may be due to differences in the myometrium of obese women resulting in decreased contraction strength. There have been only a few studies comparing the efficacy of the various cervical ripening agents in obese women and at this point no recommendation can be made as to what method may result in the greatest chance of a successful induction.

Predictors of failed trial of labor among women with an extremely obese body mass index

OBJECTIVE

The purpose of this study was to examine predictors associated with cesarean delivery (CD) among extremely obese women undergoing a trial of labor (TOL).

STUDY DESIGN

Using a delivery database, we identified all pregnant women delivering at our institution from Jan. 1, 2008, through July 31, 2010, weighing >275 lb at the time of delivery who attempted a TOL with a singleton gestation >34 weeks' gestation. Demographic and obstetrical factors were compared for those having a successful vaginal delivery to those having a CD.

RESULTS

During the study period, there were 357 pregnant women who weighed >275 lb (all with body mass index [BMI] >40 kg/m(2)), and among these, 248 (69.5%) attempted a TOL. Women having a CD had a greater BMI (51.6 vs 49.9 kg/m(2), P = .038), were less likely to be parous (32.2% vs 65.8%, P < .0001), and were more likely to be induced (80.5% vs 57.8%) compared to those having a vaginal delivery. Using a multivariable logistic regression model, among nulliparous women, maternal age, parity, and cervical dilation at time of admission were independent predictors for CD. Furthermore, an increase in BMI of 10 kg/m(2) was associated with a 3.5 increased odds (P = .002) for CD.

CONCLUSION

Among nulliparous extremely obese women attempting a TOL, BMI was an independent predictor of CD, with the rate of CD increasing further with increasing BMI. The underlying mechanisms for failed TOL in the setting of maternal obesity remain largely unknown.